bug#29689: Commit 1d008d9f8c44dfdb808235d451b72f255e72f103 gives derivation errors.

[Rutger Helling]

Commit 1d008d9f8c44dfdb808235d451b72f255e72f103 results in errors of the 
following type in guix environment/package/system etc.

guix system: error: build failed: derivation 
`/gnu/store/cmw2cl4fg1q5q5pdp7cbs6fd7yyg9phz-guile-2.0.14.drv' has 
incorrect output 
`/gnu/store/06y7gah4s74w66v0ba8pfd0azhkdkxhw-guile-2.0.14', should be 
`/gnu/store/p50qidshrjvvd7d1955navjkijr2fg32-guile-2.0.14'

Reverting the commit makes things work again.